Nitrogen-defect aggregation characteristics of some Australasian diamonds; time-temperature constraints on the source regions of pipe and alluvial diamonds

The proportions of aggregated N substitutional defects, platelet intensities, and bulk N contents have been quantitatively determined in type la diamonds from the Argyle and Ellendale olivine lamproite pipes (Kimberley block of northwest Australia) and alluvial deposits in western and central Kalimantan (Indonesia) and Copeton (eastern Australia) using Fourier transform infrared (FTIR) microscopy.

The extent of conversion of A defects (a pair of substitutional N atoms) to the aggregated B form is a function of the temperature history, mantle residence time (t_MR), and N concentration of diamond. We have used Argyle eclogitic diamonds, which have well-constrained equilibration temperatures (average ≈1255 °C) and short mantle residency (0.4 Ga), together with experimental data to refine the values of the activation energy (E_a) and Arrhenius constant (A) for the A → B reaction. We obtain E_a/R = 8.16 ± 0.13 x 10⁴ K (E_a = 7.03 eV) and ln (A/atomic ppm^–1 s^–1) = 13.51. Kinetic modeling shows that the reaction is sensitive to time-averaged temperatures (designated T_NA) in the range 1050-1300 °C and that such temperatures are not necessarily equivalent to those determined by silicate inclusion geothermometry.

The results show that the Ellendale eclogitic diamonds were resident in cool lithosphere and have not experienced the higher temperatures typical of Argyle diamonds. Constraints from inclusion geothermometry indicate that Ellendale eclogitic diamonds must be younger than Argyle diamonds, and a Phanerozoic age is suggested. Copeton diamonds comprise a yellow high-N group (>500 ppm N) and a colorless low-N group (<400 ppm N). They are constrained to a series of moderate temperature isotherms (T_NA ≈ 1070–1145 °C, for t_MR = 1.6 Ga) and have an extent of N-defect conversion consistent with long-term mantle storage. Remarkably, some of the Kalimantan alluvial and Ellendale peridotitic diamonds lie on the tightly constrained highest temperature (1145 °C) Copeton isotherm, suggesting these diamonds may have had a common origin in ancient Gondwanaland lithosphere that has been dispersed by more recent tectonic processes. A case is made for diamond survival in remnant subcontinental lithosphere underlying southeast Asian and eastern Australian microcontinental blocks that separated from the northeastern Gondwanaland margin during the Paleozoic.

Inasmuch as the N-defect characteristics of diamond reflect a unique mantle thermal history, they may prove useful in provenance studies of alluvial diamonds, in paleotectonic reconstructions, and in deducing the thermal evolution of subcontinental lithosphere, when combined with data from inclusion and xenolith geothermometry.